There are different views on the cause of a derailment due to the passing of a switch under the shunting unit: rail transport company or rail infrastructure company. CKBIS inspection body was assigned to provide an expert opinion to investigate the cause of the accident. For this purpose, existing documents are to be evaluated, the interlocking logic of the relay interlocking is to be analysed and the event has to be reconstructed on site. The assessment will include possible scenarios in order to consider all operational, safety and external influencing factors.